1. Field of the Invention
The present invention relates generally to an electronic toll collection system (hereinafter referred to as the ETC system in short) for an intelligent transport system (hereinafter also referred to as the ITS in short). More particularly, the present invention is concerned with a dedicated short-range communication apparatus (hereinafter also referred to as the DSRC apparatus) which is mounted on an automobile, a motor vehicle, a car or the like and which is designed to serve for disposing of payment of fee or charge or toll through cooperation with the ETC system. Hereinafter, the dedicated short-range communication apparatus will also be referred to as the DSRC apparatus or vehicle-onboard DSRC apparatus. Thus, in more concrete, the present invention concerns an improvement of the vehicle-onboard DSRC apparatus such that the receiving sensitivity thereof can be variably or adjustably set in dependence on the type or specie of the motor vehicle on which the DSRC apparatus is mounted for the purpose of suppressing or preventing error(s) which may otherwise occur in the radio communication with on-road transport managing equipment such as exemplified by the ETC system mentioned above which is installed stationarily on the ground in association with a road such as a toll road and which is usually incorporated in the intelligent transport system.
2. Description of Related Art
Heretofore, the vehicle-onboard DSRC apparatus is well known in the art which is designed to perform communication wirelessly with the on-road transport managing equipment such as the ETC system equipment installed in association with a road such as a toll road for the motor vehicles for exchanging or transferring various information with the on-road transport managing equipment or ETC system.
In general, the receiving sensitivity of the vehicle-onboard DSRC apparatus is set lower than -60.5 dBm inclusive in conformance with the stipulations of the Standards prescribed for such vehicle-onboard DSRC apparatus. Further, the non-response level is required to be lower than -70.5 dBm inclusive.
Under the circumstances, the sensitivity of the vehicle-onboard DSRC apparatus is so adjusted as to lie within a range of -60.5 dBm to -70.5 dBm in the manufacturing factory, i.e., before shipping of the vehicle-onboard DSRC apparatus. In this conjunction, it is however noted that in the stage of manufacturing of the DSRC apparatus, it is not yet known on what type or species of the motor vehicle the DSRC apparatus is destined to be mounted. For this reason, the sensitivity of the DSRC apparatus is ordinarily and generally set at an average value on the order of -65 dBm.
For better understanding of the concept underlying the present invention, description will first be made in some detail of the conventional vehicle-onboard DSRC apparatus known heretofore. FIG. 4 of the accompanying drawings is a block diagram showing schematically and generally a structure of a conventional vehicle-onboard DSRC (Dedicated Short-Range Communication) apparatus which constitutes a part of an ETC (Electronic Toll Collection) system and which is comprised of a transmitting/receiving unit for performing communication with the ETC system (typical one of the on-road transport managing equipment) via a vehicle-onboard antenna and an overhead antenna unit of the ETC system (described hereinafter) and a control unit (not shown).
Referring to FIG. 4, the vehicle-onboard DSRC apparatus 10 includes a receiver section which is composed of a vehicle-mounted antenna 11 (i.e., antenna mounted on the vehicle) for receiving a radio signal sent from the on-road transport managing equipment such as the ETC system, a low-noise amplifier 12 for amplifying the received signal, a mixer circuit 13 for converting the frequency of the received signal into a low frequency by making use of a frequency of a local oscillator (not shown), a semi-fixed variable gain amplifier 14 for amplifying the signal outputted from the mixer circuit 13, a detector circuit 15 for restoring received data from the received signal through demodulation or decoding, and a received data processing unit 16 for processing the received data.
FIG. 5 is a schematic side elevational diagram for illustrating in an outer appearance a general arrangement or configuration of a conventional ETC (electronic toll collection) system, a typical one of the on-road transport managing equipment. As can be seen in FIG. 5, automobiles or motor vehicles running on a road includes various species or types of motor vehicles such as, for example, a passenger car C1, a motor truck C2, a van and others.
In this conjunction, it is noted that the position at which the vehicle-onboard DSRC apparatus is installed differs from one to another type of the motor vehicle. By way of example, in the passenger cars C1 and motor truck C2 shown in FIG. 5, the positions at which the respective vehicle-onboard DSRC apparatuses 101 and 102 are installed differ from each other. More specifically, the height of the vehicle driving position (and hence the position of the vehicle-onboard DSRC apparatus) as measured from the ground level or road surface differs from one to another type or species of the motor vehicle, as indicated in FIG. 5 by H1 and H2 for the passenger car C1 and the motor truck C2, respectively. In addition, distances between the front tip or end of the motor vehicle and the location at which the vehicle-onboard DSRC apparatus is mounted differs in dependence on the vehicle types, as indicated by L1 and L2 for the motor vehicles C1 and C2, respectively, in FIG. 5.
In more concrete, in the case of the passenger car C1, the position at which the vehicle-onboard DSRC apparatus 101 is mounted is low because the position of the driver's seat is low. Consequently, in the passenger car C1, the height H1 of the position at which the vehicle-onboard DSRC apparatus 101 is mounted lower than the height H2 of the position at which the vehicle-onboard DSRC apparatus 102 of the motor truck C2 is mounted.
Moreover, it is noted that in the passenger car C1, the distance between the driver's seat and the front end of the car C1 (referred to as the nose distance) is long. In other words, in the passenger car C1, the distance L1 between the front end and the position at which the vehicle-onboard DSRC apparatus 102 is mounted is longer than the distance L2 between the front end of the motor truck C2 and the vehicle-onboard DSRC apparatus 102.
On the other hand, in a toll gate or the like system of a toll road, an overhead antenna unit ANT is installed as a part of the on-road transport managing equipment such as the ETC system, as can also be seen in FIG. 5. Additionally installed at a position in front of the overhead antenna unit ANT with a distance of several meters (e.g. 4 m) therefrom is a vehicle detector D for detecting the front end portion of the vehicle such as the passenger car C1, the motor truck C2 and the others. Needless to say, the vehicle detector D constitutes a part of the on-road transport managing equipment such as the ETC system.
The on-road transport managing equipment or the ETC system in this illustrative case is so designed as to perform communication with the vehicle-onboard DSRC apparatus such as 101; 102 via the vehicle-mounted antenna 11 and the overhead antenna unit ANT, starting from the time point at which the front end of the motor vehicle such as the passenger car C1, the motor truck C2 or the like has just traversed the vehicle detector D.
FIG. 6 is a characteristic diagram for illustrating profiles of the electric field intensities (received powers) of the radio signal emitted from the overhead antenna ANT and received by the vehicle-onboard DSRC apparatus as a function of the positions of the DSRC apparatus, wherein the position of the vehicle-onboard DSRC apparatus is taken along the abscissa with the electric field intensity being taken along the ordinate. At this juncture, it should be mentioned that although only the difference in the height H of the position of the vehicle-onboard DSRC apparatus is currently concerned, the characteristic of the electric field intensity differs in dependence on the difference in the distance L from the front end of the motor vehicle to the position of the vehicle-onboard DSRC apparatus.
Referring to FIG. 6, a single-dotted curve represents the characteristic curve in the case where the height H is 1 m (e.g. typified by the passenger car C1 or the like), while a solid-line curve represents the characteristic curve when the height H is 2 m (e.g. typified by the motor truck C2 or the like).
The electric field intensity of the radio signal (received signal power) radiated from the overhead antenna unit ANT exhibits such profile as illustrated in FIG. 6. More specifically, at the time point when the front end of the motor vehicle has reached the position at which the vehicle detector D of the on-road transport managing equipment or ETC system is installed, communication between the vehicle-onboard DSRC apparatus and the on-road transport managing equipment is started, as mentioned previously. In that case, the electric field intensity of the radio signal as received by the DSRC apparatus is at maximum at the time point immediately before the motor vehicle has reached the position at which the overhead antenna unit ANT is installed.
Further, as can be seen from the single-dotted curve and the solid-line curve shown in FIG. 6, the electric field intensity prevailing at the communication starting time point, i.e., at the time when the motor vehicle has passed by the position where the vehicle detector D is mounted differs in dependence on the height position H of the vehicle-onboard DSRC apparatus. For example, the electric field intensity becomes lower as the height H increases, the reason for which can be explained as follows.
Namely, the electromagnetic wave signal or radio signal is emitted from the overhead antenna unit of the on-road transport managing equipment from the position higher than the top of the motor vehicle and the area or range covered by the overhead antenna unit ANT, i.e., the coverage of the overhead antenna unit ANT, is restricted toward the road surface. Consequently, the radio wave radiated from the overhead antenna unit ANT becomes more difficult to be received as the height H increases particularly in the vicinity of the vehicle detector D.
More specifically, the position of the vehicle-onboard DSRC apparatus at the time point at which the front end or tip portion of the motor vehicle has transversed the vehicle detector D (i.e., at the time point when the communication between the on-road transport managing equipment or ETC system and the vehicle-onboard DSRC apparatus is started) differs in dependence on the nose length L (length of the bonnet) of the motor vehicle and the height H of the vehicle-onboard DSRC apparatus and hence in dependence on the vehicle types or species such as the passenger car C1, the motor truck C2, the van and others). Consequently, the electric field intensity of the radio signal received at the time point at which the communication is started becomes different in dependence on the height position H of the vehicle-onboard DSRC apparatus and the nose length L.
Under the circumstances, there may arise such situation that the receiving sensitivity of the vehicle-onboard DSRC apparatus becomes insufficient at the time point when the front end of the motor vehicle has just transversed the vehicle detector D, making it impossible to perform the communication, although it depends on the preset value of the receiving sensitivity of the vehicle-onboard DSRC apparatus.
By way of example, let's assume that the receiving sensitivity of the vehicle-onboard DSRC apparatus is set at -65 dBm. In that case, when the vehicle concerned is the passenger car C1, the radio signal of the electric field intensity higher than -65 dBm (e.g. on the order of -58 dBm) can be received by the DSRC apparatus mounted on the passenger car C1 at the time point when the front end of the motor vehicle has reached the vehicle detector D. By contrast, when the vehicle concerned is the motor truck C2, the electric field intensity of the radio signal received by the DSRC apparatus mounted on the motor truck C2 is lower than -65 dBm (e.g. on the order of -68 dBm), as a result of which the communication between the DSRC apparatus of the motor truck C2 and the on-road transport managing equipment (ETC system) is rendered impossible (see FIG. 6).
On the other hand, when the receiving sensitivity of the DSRC apparatus is set high (e.g. on the order of -70.5 dBm), communication with the overhead antenna unit ANT will be started already at a position which precedes to the vehicle detector D in case the motor vehicle concerned is the passenger car C1.
As is apparent from the above, when the receiving sensitivity of the vehicle-onboard DSRC apparatus is set at an unnecessarily high level, such unwanted situation may arise that erroneous communication is started between the DSRC apparatus and the on-road transport managing equipment (ETC system) before the motor vehicle equipped with the DSRC apparatus transverses the vehicle detector D when a motor vehicle running ahead which is not equipped with the DSRC apparatus transverses the vehicle detector D. In that case, communication with the on-road transport managing equipment will be started instantaneously when the motor vehicle running ahead and equipped no DSRC apparatus passes by the vehicle detector D before the motor vehicle which is equipped with the DSRC apparatus transverses the vehicle detector D, as a result of which the motor vehicle running ahead may erroneously be identified as the vehicle equipped with the DSRC apparatus, incurring erroneous communication between the motor vehicle running ahead and the on-road transport managing equipment or ETC system.
When the situation described above occurs really, the communication with the on-road transport managing equipment (ETC system) has already been completed at the time when the vehicle equipped with the DSRC apparatus actually transverses the vehicle detector D, incurring undesirably occurrence of the communication error.
As will now be appreciated, since the mounting position of the vehicle-onboard DSRC apparatus differs in dependence on the types or species of the motor vehicles, difference makes appearance in the electric field intensity of the radio signal received by the vehicle-onboard DSRC apparatus at the time point when the front end portion of the motor vehicle transverses the vehicle detector D, which may possibly incur the communication error.
In the present state of the art, the communication error mentioned above can ordinarily be suppressed by setting properly the directivity of the overhead antenna unit ANT, location/orientation thereof and the communication protocol as adopted. However,when the conditions for the radio signal change due to weather, traffic statuses in the vicinity of the toll gate and for other reasons, communication error may take place unwantedly, giving rise to a problem.
In this conjunction, it should also be added that when the communication error such as mentioned above occurs actually, the automatic toll keeper function will be invalidated, although such serious problem as erroneous payment or the like can be excluded owing to the recovery function of the communication protocol and the on-road transport managing equipment. In that case, however, the driver is forced to park once his or her car at the toll gate for paying the toll.
Furthermore, it should be mentioned that in the communication area, there exists a communication-invalidated area (area where the electric field intensity of the radio signal emitted from the overhead antenna unit is very low) due to the side robes of the output of the overhead antenna unit ANT in addition to the reception irregularities due to difference of the motor vehicle type. In such communication area, the reception characteristic itself does change, which causes the communication error to be more likely to occur.
As will now be understood from the foregoing, in the conventional DSRC apparatus, the receiving sensitivity thereof is set at a predetermined constant level uniformly among the motor vehicles regardless of difference in the type or species thereof. Consequently, when the receiving sensitivity of the vehicle-onboard DSRC apparatus is lower than a prescribed level, the radio signal radiated from the overhead antenna unit of the on-road transport managing equipment such as the ETC system can not be received by the vehicle-onboard DSRC apparatus, rendering the communication impossible, whereas when the receiving sensitivity is set higher than the prescribed level, erroneous communication may take place before the motor vehicle enters the communication area, giving rise to a problem that the function of the vehicle-onboard DSRC apparatus can not effectively be utilized, to a disadvantage.